Kinetics and mechanism of heterogeneous palladium-catalyzed coupling reactions of chloroaryls in water

Coupling reactions of substituted chlorobenzenes to biphenyls catalyzed by palladium on carbon are performed in water using sodium hydroxide and sodiu m formate in the presence of a surface active agent. Thus, chlorobenzene, p -chloro-o-xylene, p-chloro-1,1,1-trifluorotoluene, p-chloroanisole, and p-c hlorotoluene are coupled under moderate conditions to the respective biaryl s. A competing reduction process occurs (eg. chlorobenzene is reduced to be nzene), which can be minimized by altering conditions. The relationship of product selectivity to reaction temperature, formate concentration, base co ncentration, and surfactant type is examined. The roles of formate, Pd cata lyst, and surfactant are discussed. It is proposed that the reduction is de pendent on the participation of palladium hydride [Pd2+(H-)(2)], while the coupling occurs via single electron-transfer from Pd-0 to the substrate, wi th subsequent decomposition of the chloroaryl radical anions to obtain aryl radicals and chloride anions. This mechanism is supported by experiments w ith stoichiometric and sub-stoichiometric amounts of palladium which indica te that selective coupling can occur also in the absence of hydrogen (provi ding that reduced palladium pd(0), is present in sufficient amount), and by kinetic investigations which indicate that the coupling is actually a firs t-order reaction, for which the rate-determining step may be the dissociati on of the chloroaryl radical anion.